Solar power generation (photovoltaic power generation) in which a light energy is converted into an electric energy by a photoelectric transfer effect has been extensively used as a means for attaining a clean energy. With the recent enhancement in photoelectric transfer efficiency of photovoltaics, solar power generation systems have been installed even in a large number of individual houses. In order to employ these solar power generation systems as a practical power source, there has been used a solar battery module having a plurality of solar battery cells which are connected in series to each other.
Since the solar battery module is used under high-temperature and high-humidity environmental conditions for a long period of time, a film for a backsheet of photovoltaics as a constitutional member of the photovoltaics is also required to exhibit a long-term durability. For example, there has been proposed the technique in which a based film is used as the film for a backsheet of fluorine-photovoltaics (Patent Document 1). In the thus proposed technique, it is described that the fluorine-based film is previously subjected to heat treatment to reduce a shrinkage rate of the fluorine-based film, so that there can be obtained the effect of preventing deterioration in properties including weather resistance and water resistance when subjected to vacuum lamination with ethylene vinyl acetate (hereinafter occasionally referred to merely as “EVA”) as a sealing material as well as the effect of enhancing a yield of the film.
However, the fluorine-based film has a relatively low rigidity and therefore tends to be difficult to handle. Further, since the fluorine-based film is expensive as compared to generally used resin films, there tends to arise such a problem that a solar battery module produced using such a fluorine-based film is also expensive.
Conventionally, it has been attempted to use a polyester-based film as the film for a backsheet of photovoltaics in place of the above fluorine-based film. In general, as is known in the art, when using the polyester-based film under high-temperature and high-humidity environmental conditions, there tends to occur such a problem that the polyester-based film suffers from hydrolysis at an ester bond moiety in a molecular chain thereof, so that mechanical properties of the polyester-based film tend to be deteriorated. In consequence, in view of such a case that the polyester-based film for the solar battery module is used outdoors over a long period of time (for example, over 20 years) or under high-temperature and high-humidity environmental conditions, there have been made various studies for suppressing occurrence of hydrolysis of the polyester.
It is known that the rate of hydrolysis of a polyester becomes higher as a content of a carboxyl end group in a molecular chain of the polyester is increased. Therefore, there has been proposed such a technique that by adding a compound capable of reacting a carboxylic acid, the amount of a carboxyl group being present in a terminal end of a molecular chain of a polyester is reduced to thereby enhance a hydrolysis resistance of the polyester (Patent Documents 2 and 3). However, addition of the above compound tends to induce gelation of the material upon melt extrusion step or a material recycling step in a film formation process to generate foreign matters, resulting in deteriorated quality and high product costs, as well as high burdens on environments. In addition, the polyester-based film has a high shrinkage rate and therefore is unsuitable as films used for production of solar battery modules.